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That means there is no 24 volts AC detected across the R and C terminals at the unit, check that there with a volt meter. If there is no voltage there then
What likely happened is the low voltage fuse in the furnace or airhandler blew. There typically is a SLOT STYLE 3-5AMP FUSE on the board itself inside the furnace/airhandler like this.
If that is burned out all power to the thermostat will be lost. 95% of all units use some sort of SLOT FUSE or inline BUS STYLE FUSE to protect the low voltage circuit. You need to find that and change it out. If you just bump two wires together you could have just popped it without even knowing it. Now if for some reason your unit does not use a fuse often the board will be damaged due to the short. You would need to confirm you have 24 volts to the board inside the furnace/airhandler with a volt meter to make sure though.
Note if you have a condensate pump check the float and make sure the pump is not full of water...as that could cut low voltage power to the unit as well if the pump is not working and full of water.
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